1. Field of the Invention
This invention pertains to the field of computer chip capacitor electrode termination. More particularly, this invention is a unique method of profiling the wet paste layer to control the border the termination paste makes with the computer chip capacitor (chip) to insure that there are no areas where the paste dips toward the edge of the chip or thins out to adversely affect the ultimate properties of the finished chip.
2. Description of the Prior Art
Computers and other electronic appliances are growing in importance in our daily lives at a rate that appears to be reaching exponential proportions. These electronic devices reach all ages and areas of endeavor. Infants are exposed to computers through electronically-based games that teach musical tones and image recognition, such as cows, trucks, bananas, etc. Preschoolers play games on computers that lay the basis for later learning math, reading and geography. Elementary, Junior High, High School and College students take classes in computer theory, programming and operation. For those who entered the work force before computers became a part of the school curriculum, there are seminars, home-study courses and public television courses that introduce one to computers, computer operation and software operation. Virtually every business in the United States of America uses computers somewhere in its operation.
The use of computers to undertake business functions and educational activities has created a demand for more computer capacity and faster processing times. What started out as a simple Commodore.RTM. 64 computer with black/white monitor (having 64 k of memory utilizing a very short program) now is a 1-32 megabyte RAM/5 Gigabyte hard drive computer with color monitor, offering programs that include moving pictures and music. In addition, other electronic devices, such as VCRs, television sets, cameras, camcorders, auto alarms, radios and the like are using more and more computer hardware to become more user-friendly with expanded capabilities. Competition among various electronic companies has forced computer and computer-accessory manufacturers to design more powerful and more capable circuitry. Just one indication of the astonishing rate of computer development is seen in the case of hard drives. In the past 15 years, hard drive storage capacity has increased at an average annual rate of a factor of three. Where in 1982 a standard hard drive was 10 megabytes, it has grown to 5 Gigabytes in 1997.
At the center of all this activity is a device so small that one could easily miss its existence, if not careful to look for it. It is a miniature capacitor, of a general size of 0.040 inches long with a square or rectangular cross-section 0.020 inches on a side, with spaced-apart flat end surfaces. These devices are constructed of multiple layers of electrically conductive material held apart by a ceramic dielectric matrix that is fired to produce a hard, minute device much smaller than a grain of rice. Used by the thousands in computer circuitry, these capacitors, called "chips" by those in the trade, are in great demand.
In the circuitry, they are soldered directly to the computer circuit board on their flat side surface and often resemble miniature bumps on the circuit board. For this special soldering technique, the end or edge surfaces of the conductive layers in the chips are coated with a silver paste and fired, at high heat, to cure or set the silver paste to a coating that is easily soldered to a specially designed copper "flat" located on the computer circuit board. To coat these ends of the chips there has been invented hand-operated tools with which to position the chips for coating with a wet, curable compound, reference U.S. Pat. Nos. 4,381,321; 4,393,808; 4,395,184; and 4,526,129.
In this prior art practice, a layer of the wet paste is cast on a planar surface, the top of the layer is scraped to a level surface of desired thickness (usually 0.005 inches), and then the chips are dipped into the layer to apply a coat of the paste in a band along the chip edge to a depth of 0.005 inches. The chip with its band of wet paste is then passed through a dryer and then the chip reversed and the opposite, uncoated edge dipped in another, similar layer of paste and then dried to complete the termination process.
A problem has developed in that the surface tension of the paste causes a momentary "belly" or dip to form along the entire length of the chip edge when it is dipped in the paste and about its edges with the lowest part of the dip being at the middle of the chip. Surface tension forces and the wettability of the paste blend out much of this belly over the next few seconds, however the surface tension at the corners of the chip remains unaffected, due in large part to the high tension at the sharp corners. The border of paste thereafter formed on the chip electrode often displays a distinct lower area or dip at the corner edges. Such unevenness in the border creates problems of current density coming into the chip from the underlying circuit tracing to which the chip is soldered. Additionally, the unevenness in the border at the corners of the chip results in less silver at the corners which makes the chip susceptible to damage during testing and/or mounting. Still further, when the border is uneven, the higher border at the middle of a chip whose corners are at the proper border height represents more silver than is necessary to terminate the chip and, when multiplied by the large number of chips that have this malady, represents a direct loss of value in the chip.
Changing the density of the paste, adding surface tension modifiers to the paste, and other such potential remedies have not proven adequate because of the effects they have on other properties of the chip.